5009
From the thermochemical data given below, what can you say about the feasibility of the following reactions to proceed as written? Show your calculations.
(a)
and
(b)
Average bond energies
|
Bond |
∆H (Kcal m–1) |
Bond |
∆H (Kcal m–1) |
|
H–H |
104.2 |
C–I |
51 |
|
C–H |
99 |
I–I |
36.5 |
|
H–I |
71.4 |
C–F |
116 |
|
H–F |
135.9 |
F–F |
37.5 |
5114
Using the bond dissociation energies
provided below, answer the following questions, and given that
|
Reaction |
∆H°
(Kcal m–1) at 25° |
|
Cl + CH4 → HCl +
CH3 |
+1.0 |
|
CH3 +
Cl2 → CH3Cl +
Cl |
-25.7 |
|
Cl2 → 2Cl |
+58.2 |
(a)
Draw a potential energy diagram for the free radical chain monochlorination of methane by chlorine if the radical chain length
is very large.
(b)
Using the tabulated bond dissociation energies below, calculate the
enthalpy for the monochlorination of methane using
chlorine. Does this number agree with
that you obtain from the stepwise reaction in part (a) above?
(c)
What is the source of
differences between average bond energies and bond dissociation
energies? (Assume that they are
tabulated for the same temperature, pressure, etc).
[Bond dissociation energy: Cl–Cl, 58.0
Kcal m–1; H–Cl, 103.1 Kcal m–1;
CH3–H,
102.0 Kcal m–1.]